Magnesium-containing Zintl phase compounds Yb 14 MgSb 11 and Ca 14 MgSb 11 have been prepared by annealing the mixture of the elements at 1075−1275 K. These compounds are isostructural with the Zintl compound Ca 14 AlSb 11 and crystallize in the tetragonal space group I4 1 /acd (Z = 8). Singlecrystal X-ray data (90 K) were refined for Yb 14 MgSb 11 [a = 16.625(9) Å, c = 22.24(2) Å, V = 6145(8) Å 3 , and R1/wR2 (0.0194/0.0398)] and Ca 14 MgSb 11 [a = 16.693(2) Å, c = 22.577(5) Å, V = 6291(2) Å 3 , R1/wR2 (0.0394/0.0907)].This structure type has been shown to be highly versatile with a large number of phases with the general formula A 14 MPn 11 (A = Ca, Sr, Ba, Yb, Eu; M = Mn, Zn, Nb, Cd, Group 13 elements; Pn = Group 15 elements). The two compounds reported in this paper are the first Mg-containing analogs. Replacing M with Mg, which is divalent with no d-orbitals, probes electronic structure and properties of this structure type. Mg 2+ is well-known to prefer tetrahedral geometry and allows for integration of the properties of a main group analog isoelectronic to the transition metal (Mn 2+

Compounds of the Yb 14 MnSb 11 structure type are the highest efficiency bulk p-type materials for high temperature thermoelectric applications, with reported figures of merit (ZTs) as high as ∼1.3 at 1275 K. Further optimization of ZT for this structure type is possible with the development of a simple synthetic route. However, this has been difficult to achieve because of the small amount of Mn required compared with Yb and Sb. A simple synthetic route for Yb 14 MnSb 11 has been developed utilizing a combination of ball milling and annealing to produce phase-pure material followed by spark plasma sintering for consolidation. The materials have been characterized by powder X-ray diffraction before and after spark plasma sintering. The stoichiometric reaction of Yb, Sb, and MnSb provides phase-pure powder by X-ray diffraction. Upon cycling to temperatures greater than 1272 K, Yb 14 MnSb 11 shows the presence of Yb 11 Sb 10 . Additional samples with 5% and 10% excess Mn were also investigated. Adding 5−10% excess Mn does not change the low temperature properties and improves the high temperature ZT, resulting in a ZT of 1.1−1.2 at 1000 K for Yb 14 Mn 1.05 Sb 11 , 30−40% improvement over that of the Sn flux reaction. The increase in ZT is attributed to optimization of the carrier concentration. These results provide a reliable method of bulk synthesis of this Zintl phase and open the way for discovery of new compounds with potential for even higher ZT.

Yb14MnSb11 is a member of a remarkable structural family of compounds that are classified according to the concept of Zintl. This structure type, of which the prototype is Ca14AlSb11, provides a flexible framework for tuning structure-property relationships and hence the physical and chemical properties of compounds. Compounds within this family show exceptional high temperature thermoelectric performance at temperatures above 300 K and unique magnetic and transport behavior at temperatures below 300 K. This review provides an overview of the structure variants, the magnetic properties, and the thermoelectric properties. Suggestions for directions of future research are provided. One active research area is to systematically explore more complex compositions such as Ca11Sb10, K4Pb9, Na8Si46, Ca14AlSb11 and KBa2InAs3. 3-7 The other direction is to replace the alkaline earth metals with divalent rare earth elements (Sm, Eu and Yb) along with the introduction of transition metals into structures, typically replacing the less electronegative metalloid in the anionic framework. 8-10 Combinations of these two directions led to compounds such as Yb14MnSb11, Pr4MnSb9, Eu10Mn6Sb13, Yb9Zn4+xBi9 and Cs13Nb2In6As10. 11-16 The complexity of compositions can be combined with a small flexibility in electron counting. For example, Yb14MnSb11 and Yb9Zn4+xBi9 do not strictly follow the Zintl-Klemm concept. Yb14MnSb11 has Mn 2+ instead of a group 13 element such as in Ca14AlSb11 and therefore is electron deficient, 17 and Yb9Zn4+xSb9 has interstitial Zn atoms which can be compositionally varied to achieve specific properties. 18 At the same time, the total number of valence electrons within an identical Zintl phase structure type with different elements may also vary slightly but the variance can be quite small and limited for many structure types that can be described by the Zintl concept. Therefore, with the introduction of transition elements, new electronic properties are possible, but complete transfer of electrons and clear counting of valence electrons remains a criterion for describing transition and rare earth metal containing Zintl phase compounds. Binary Zintl phase compounds which have compositions of simple ratios of elements usually adopt the structures of known oxides or halides, in which anions and cations are isolated in the structure with no covalent bonding. 2, 19 Both isolated anions, polyanions or clusters in Zintl phase compounds can provide complex compositions such as those represented by Ca11Sb10 and K4Pb9. 3, 4 Polyanions or clusters are formed to compensate for lack of enough electrons from the electropositive element to satisfy valence to form a simple one atom anion. Sb forms Sb-Sb single bonds in the Ca11Sb10 structure type resulting in Sb2 4and Sb4 2polyanions in the structure. 3 The Zintl electron counting provides the following charge balanced scenario: 11Ca 2+ + 4Sb 3-+ 2Sb2 4-+ Sb4 2-. Two types of clusters exist in K4Pb9 with the same formal oxidation state: a monocapped square antiprism and a tr...

We present a systematic study of the structural and magnetic properties of two branches of the rare earth Tripod Kagome Lattice (TKL) family A2RE3Sb3O14 (A = Mg, Zn; RE = Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb; here, we use abbreviation A-RE, as in MgPr for Mg2Pr3Sb3O14), which complements our previously reported work on MgDy, MgGd, and MgEr 15 . The present susceptibility (χ dc , χac) and specific heat measurements reveal various magnetic ground states, including the non-magnetic singlet state for MgPr, ZnPr; long range orderings (LROs) for MgGd, ZnGd, MgNd, ZnNd, and MgYb; a long range magnetic charge ordered state for MgDy, ZnDy, and potentially for MgHo; possible spin glass states for ZnEr, ZnHo; the absence of spin ordering down to 80 mK for MgEr, MgTb, ZnTb, and ZnYb compounds. The ground states observed here bear both similarities as well as striking differences from the states found in the parent pyrochlore systems. In particular, while the TKLs display a greater tendency towards LRO, the lack of LRO in MgHo, MgTb and ZnTb can be viewed from the standpoint of a balance among spin-spin interactions, anisotropies and non-Kramers nature of single ion state. While substituting Zn for Mg changes the chemical pressure, and subtly modifies the interaction energies for compounds with larger RE ions, this substitution introduces structural disorder and modifies the ground states for compounds with smaller RE ions (Ho, Er, Yb).

Thermoelectric materials are a unique class of compounds that can recycle energy through conversion of heat into electrical energy. A new 21–4–18 Zintl phase has been discovered in the Yb–Mn–Sb system with high performance in the mid-to-high temperature regime. The efficiency of the Yb21Mn4Sb18 results mainly from its large Seebeck coefficient (∼290 μV K–1 at 650 K) and extremely low thermal conductivity (∼0.4 W m–1 K–1). The complex crystal structure has been studied through single crystal X-ray diffraction, synchrotron powder X-ray diffraction, and pair distribution function (PDF) analysis using time-of-flight neutron diffraction revealing positional disorder on several sites. Electronic structure calculations of the band structure and the partial spin-density of states reveal that states near the Fermi level are contributed mostly by the Mn and Sb atoms that participate in the [Mn4Sb10]22– motif of the structure. The band structure confirms the p-type semiconducting nature of this material. The optimization of the hole carrier concentration was tuned according to a single parabolic band model through Na doping on the Yb site (Yb21–x Na x Mn4Sb18, x = 0, 0.2, 0.4) showing an improvement in zT over the whole temperature range. A maximum zT ≈ 0.8 at 800 K is obtained for the x = 0.4 sample and increases the ZTavg from 0.34 to 0.49 (over the entire temperature range) compared to the undoped sample.

ThezTof Yb14MnSb11is improved by the introduction of a light rare earth element, RE3+(RE = Pr, Sm) with partially filled f-levels. The carrier concentration is reduced upon substituting RE3+for Yb2+, adding one electron to the system and improving thezTvalues 30–40% over that of the pristine material.

Single crystals of Yb14-xRExMnSb11 (0 < x < 0.6, RE = Pr, Nd, Sm, and Gd) were synthesized by Sn flux. The compounds are iso-structural with Ca14AlSb11 (I41/acd), and their compositions were determined by wavelength-dispersive spectroscopy. Yb14MnSb11 is described as a partially screened d-metal Kondo system with the isolated [MnSb4](9-) tetrahedral cluster having a d(5) + hole configuration that results in four unpaired electrons measured in the ferromagnetically ordered phase. All of the Yb atoms in Yb14MnSb11 are present as Yb(2+), and the additional RE in Yb14-xRExMnSb11 is trivalent, contributing one additional electron to the structure and altering the magnetic properties. All compounds show ferromagnetic ordering in the range of 39-52 K attributed to the [MnSb4](9-) magnetic moment. Temperature-dependent DC magnetization measurements of Yb14-xPrxMnSb11 (0.44 ≤ x ≤ 0.56) show a sharp downturn right below the ferromagnetic transition temperature. Single-crystal neutron diffraction shows that this downturn is caused by a spin reorientation of the [MnSb4](9-) magnetic moments from the ab-plane to c-axis. The spin reorientation behavior, caused by large anisotropy, is also observed for similar x values of RE = Nd but not for RE = Sm or Gd at any value of x. In Pr-, Nd-, and Sm-substituted crystals, the saturation moments are consistent with ∼4 unpaired electrons attributed to [MnSb4](9-), indicating that local moments of Pr, Nd, and Sm do not contribute to the ferromagnetic order. In the case of RE = Pr, this is confirmed by neutron diffraction. In contrast, the magnetic measurements of RE = Gd show that the moments of Gd ferromagnetically order with the moments of [MnSb4](9-), and reduced screening of moments on Mn(2+) is evident. The sensitive variation of magnetic behavior is attributed to the various RE substitutions resulting in different interactions of the 4f-orbitals with the 3d-orbitals of Mn in the [MnSb4](9-) cluster conducted through 5p-orbitals of Sb.

No abstract